1. Field of the Invention
The present invention relates to edge reflection type surface acoustic wave devices and edge reflection type surface acoustic wave filters using a Shear Horizontal type surface acoustic wave, and more specifically, to edge reflection type surface acoustic wave devices and edge reflection type surface acoustic wave filters including a divided interdigital transducer (IDT).
2. Description of the Related Art
Recently, edge reflection type surface acoustic wave devices using a Shear Horizontal (SH) type surface acoustic wave, such as the Bleustein-Gulyaev-Shimizu (BGS) wave or the Love wave, have attracted attention. Such edge reflection type surface acoustic wave devices include an IDT provided on a surface acoustic wave substrate and an SH-type surface acoustic wave is reflected between two opposing edges of the surface acoustic wave substrate. Accordingly, since it is not necessary to provide a reflector outside of the IDT in a surface acoustic wave-propagating direction, the dimensions of the surface acoustic wave devices can be reduced.
In Japanese Unexamined Patent Publication No. 5-291869, an edge reflection type surface acoustic wave device in which an IDT is disposed on a surface acoustic wave substrate and the IDT is divided into a plurality of sub-IDT portions is provided. In this device, since the IDT is divided such that the plurality of sub-IDT portions are arranged along a surface acoustic wave-propagating direction, impedance can be enhanced and capacitance can be reduced.
However, in the edge reflection type surface acoustic wave device described in Japanese Unexamined Patent Publication No. 5-291869, when the IDT is divided into the plurality of sub-IDT portions, undesired ripples (spurious response) can occur in frequency characteristics. Thus, it is necessary to select the number of the pairs of electrode fingers of the IDT in order to prevent such ripples.
In other words, since the number of the pairs of the electrode fingers of the IDT is limited, it is very difficult to obtain various frequency characteristics without generating undesired ripples.
In order to overcome the problems described above, preferred embodiments of the present invention provide an edge reflection type surface acoustic wave device and an edge reflection type surface acoustic wave filter using an SH-type surface acoustic wave, in which an IDT is divided into a plurality of sub-IDT portions, and undesired ripples occurring in frequency characteristics are minimized even when the number of the pairs of electrode fingers of the IDT is changed to obtain various frequency characteristics, so that excellent resonant characteristics and filter characteristics are achieved.
In the edge reflection type surface acoustic wave device according to a preferred embodiment of the present invention, the IDT is divided into a plurality of sub-IDT portions and electrode fingers located closest to each other between adjacent sub-IDT portions are at the same potential so that excitation does not occur between these electrode fingers. When it is assumed that the outermost widths of the IDT in the surface acoustic wave-propagating direction is substantially equal to the widths of the remaining electrode fingers, the opposing two edges of the surface acoustic wave substrate are arranged such that a distance between the opposed two edges is not an integral multiple of xcex0/2, which is achieved by cutting the edges at locations inside of the center of the outermost electrode finger in the surface acoustic wave-propagating direction. Thus, the frequency of the (2Nxe2x88x922) mode in the frequency spectrum of the IDT obtained due to the dividing of the IDT is in proximity to the frequency of a mode determined by the distance between the opposing two edges, with the result that unnecessary ripples are effectively minimized.
Therefore, with the divided IDT, it is possible to provide an edge reflection type surface acoustic wave device having excellent resonant characteristics, in which impedance and capacitance are increased, but also there is almost no influence due to limitations on the number of the pairs of electrode fingers.
In the edge reflection type surface acoustic wave device according to another preferred embodiment of the present invention, there is provided a structure in which the IDT is divided into a plurality of sub-IDT portions and electrode fingers located closest to each other between adjacent sub-IDT portions are connected to different potentials so that double excitation is generated between the electrode fingers. When it is assumed that the outermost widths of the IDT in the surface acoustic wave-propagating direction are equal to the widths of the remaining electrode fingers before the substrate is cut, the opposing two edges of the surface acoustic wave substrate are arranged such that the edges are cut at locations outside of the center of the outermost electrode finger in the surface acoustic wave-propagating direction so that a distance between the opposite edges of the substrate is not an integral multiple of xcex0/2. As a result, the frequency of the (2Nxe2x88x922) mode on the frequency spectrum of the IDT obtained due to the dividing of the IDT is in proximity to the frequency of a mode determined by the distance between the opposing two edges, with the result that unnecessary ripples are effectively minimized.
Therefore, with the divided IDT, it is possible to provide an edge reflection type surface acoustic wave device having excellent resonant characteristics, in which impedance and capacitance are increased, and there is almost no influence due to limitations on the number of the pairs of electrode fingers.
In addition, in each of the edge reflection type surface acoustic wave devices in accordance with preferred embodiments of the present invention, when the positions a of the edges are within a range of about xc2x10.036xcex0 satisfying the equation (1), unnecessary ripples are suppressed to be about 0.5 dB or lower, with the result that even more improved resonant characteristics are achieved.
Furthermore, when the positions a of the edges are within a range of about xc2x10.018xcex0 satisfying the equation (1), the magnitudes of the above ripples is about 0.2 dB or lower, so that even more improved resonant characteristics are achieved.
Furthermore, similar advantages can be obtained if the outermost electrode fingers in the surface acoustic wave-propagating direction are arranged such that the outermost electrode fingers extend to the edges defined by the edges and the upper surface of the surface acoustic wave substrate.
As described above, preferred embodiments of the present invention provide the edge reflection type surface acoustic wave device in which the divided IDT having a plurality of sub-IDT portions is disposed on a surface acoustic wave substrate. However, when a plurality of edge reflection type surface acoustic wave devices in accordance with preferred embodiments of the present invention are prepared and arranged to define a filter by coupling the plurality of edge reflection type surface acoustic wave devices, impedance and capacitance are greatly increased by dividing the IDT. As a result, edge reflection type surface acoustic wave devices having different impedance and capacitance are easily formed. Therefore, for example, when a ladder-type filter is constructed, an appropriate structure including series-arm resonators and parallel-arm resonators is produced by using the edge reflection type surface acoustic wave device in accordance with preferred embodiments of the present invention, with the result that excellent filter characteristics are easily obtained.
The edge reflection type surface acoustic wave device in accordance with preferred embodiments of the present invention can be used as a band filter. In this case, unnecessary ripples are minimized and excellent filter characteristics are thereby be obtained.
For the purpose of illustrating the invention, there is shown in the drawings several forms which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.